Simon's fears are about people, not the island. In this way, his fears are more real and true than the other boys. They fear a mythical beast; he fears the beast within the other boys.
When the fire goes out and Ralph confronts Jack, we read: "Simon looked now, from Ralph to Jack, ... and what he saw seemed to make him afraid." He senses the coming conflicts between the two boys before any such conflicts really appear.
Later, in one of their assemblies, they discuss the beast and the fears of the little'uns. Simon tells them that maybe there isn't a beast after all, but only people who will become beastlike: he says, "'Maybe it's only us.'"
When the beast on the mountain keeps them from lighting their fire, only Simon has the bravery to suggest climbing the mountain to find out what's really there. He understands that nothing on the mountain could possibly hurt them as badly as they could hurt each other if they don't get rescued soon.
When Simon talks to the beast, it says to him, "'You knew, didn't you? I'm part of you? ... I'm the reason why it's no go? Why things are what they are?'" Simon understands that the beast isn't a physical being, but a spiritual manifestation inside the boys, turning them into savages.
In Golding's "Lord of the Flies," Simon is described as having eyes "...so bright they had deceived Ralph into thinking him delightfully gay and wicked." This allows Simon to have some kind of truth; to reveal what is the truth in perceptions of the island. Therefore, he can not have any fear of the island, as that would be in conflict with the purpose of his character.
Simon stands up for Piggy when Jack knocks his glasses off, and when he tells Ralph that " You'll get back to where you came from," he has a deeper understanding of what will happen to the characters.
Simon is cast as the one person who can be revealed the truth to. When Simon thinks about the beast, "there arose before his inward sight the picture of a human at once heroic and sick." Simon knew what the beast was. And it was Golding revealing through Simon, honesty and truth at any cost.